Guest Editorial

Products with micro- and nano-scale features find widespread applications in industries including the medical, automotive, optics, electronics, energy, and biotechnology sectors. The tendency toward miniaturization and development of products in many industries exposed the limitations of established micro- and nanomanufacturing methods in terms of processing capability, speed, flexibility, accuracy, scalability, etc. To respond to these challenges, many micro- and nano-additive manufacturing (AM) technologies have been developed. Compared to traditional micro/nanofabrication methods, AM has the merits of simpler processing, shorter fabrication time, lower cost, and the capability of fabricating high aspect ratio structures and almost any complicated freeform structures. The innovation of novel AM or hybrid processes for micro/nanofabrication is a field of active research throughout the world. Accordingly, characterization, control, modeling, and simulation of the manufacturing process is in great need for achieving accurate and reliable production of micro/nanoscale features. In addition, successful production of complicated features at micro- or nano-scale requires understanding of the materials used as feedstock, and the relationships between feedstock materials, designs, processes, and properties of fabricated micro/nanoscale structures. Analysis of micro/nano-AM process performance, in terms of manufacturing flexibility, reliability, cost, and quality, is also critical to enable more applications in fields including biomedical, mechanical, sensing, actuating industries, etc.

Photopolymerization enables the printing of three-dimensional (3D) objects through successively solidifying liquid photopolymer on two-dimensional (2D) planes. However, such layer-by-layer process significantly limits printing speed, because a large number of layers need to be processed in sequence. In this paper, we propose a novel 3D printing method based on multiphoton polymerization using femtosecond Bessel beam. This method eliminates the need for layer-by-layer processing, and therefore dramatically increases printing speed for structures with high aspect ratios, such as wires and tubes. By using unmodulated Bessel beam, a stationary laser exposure creates a wire with average diameter of 100 μm and length exceeding 10 mm, resulting in an aspect ratio > 100:1. Scanning this beam on the lateral plane fabricates a hollow tube within a few seconds, more than ten times faster than using the layer-by-layer method. Next, we modulate the Bessel beam with a spatial light modulator (SLM) and generate multiple beam segments along the laser propagation direction. Experimentally observed beam pattern agrees with optics diffraction calculation. This 3D printing method can be further explored for fabricating complex structures and has the potential to dramatically increase 3D printing speed while maintaining high resolution.

Graphene possesses many outstanding properties, such as high strength and light weight, making it an ideal reinforcement for metal matrix composite (MMCs). Meanwhile, fabricating MMCs through laser-assisted additive manufacturing (LAAM) has attracted much attention in recent years due to the advantages of low waste, high precision, short production lead time, and high flexibility. In this study, graphene-reinforced aluminum alloy AlSi10 Mg is fabricated using selective laser melting (SLM), a typical LAAM technique. Composite powders are prepared using high-energy ball milling. Room temperature tensile tests are conducted to evaluate the mechanical properties. Scanning electron microscopy observations are conducted to investigate the microstructure and fracture surface of obtain composite. It is found that adding graphene nanoplatelets (GNPs) significantly increases porosity, which offsets the enhancement of tensile performance as a result of GNPs addition. Decoupling effort is then made to separate the potential beneficial effects from GNPs addition and the detrimental effect from porosity increase. For this purpose, the quantitative relationship between porosity and material strength is obtained. Taking into consideration the strength reduction caused by the increased porosity, the strengthening effect of GNPs turns out to be significant, which reaches 60.2 MPa.

A high electrical and thermal conductivity coupled with low costs make copper (Cu) an enticing alternative to aluminum for the fabrication of interconnects in packaging applications. To tap into the benefits of the ever-reducing size of transistors, it is required to increase the input/output pin count on electronic chips, and thus, minimize the size of chip to board interconnects. Laser sintering of Cu nanoparticle (NP) inks can serve as a promising process for developing these micron sized, 3D interconnect structures. However, the exact processing windows for Cu NP sintering are not well known. Therefore, this paper presents an extensive experimental investigation of the sintering processing window with different lasers including femtosecond (fs), nanosecond (ns), and continuous-wave (CW) lasers. The dependence of the processing window on Cu layer thicknesses and laser exposure durations has also been investigated. A simplified model to estimate optimum laser sintering windows for Cu NPs using pulsed lasers is presented and the predicted estimates are compared against the experimental results. Given the simplicity of the model, it is shown to provide good estimates for fluence required for the onset of sintering and the processing window for good sintering of Cu NPs.

To date, several additive manufacturing (AM) technologies have been developed for fabricating smart particle–polymer composites. Those techniques can control particle distributions to achieve gradient or heterogeneous properties and functions. Such manufacturing capability opened up new applications in many fields. However, it is still widely unknown how to design the localized material distribution to achieve desired product properties and functionalities. The correlation between microscale material distribution and macroscopic composite performance needs to be established. In our previous work, a novel magnetic field-assisted stereolithography (M-PSL) process was developed, for fabricating magnetic particle–polymer composites. In this work, we focused on the study of magnetic-field-responsive particle–polymer composite design with the aim of developing guidelines for predicting the magnetic-field-responsive properties of the composite. Microscale particle distribution parameters, including particle loading fraction, magnetic particle chain structure, microstructure orientation, and particle distribution patterns, were investigated. Their influences on the properties of particle–polymer liquid suspensions and properties of the three-dimensional (3D) printed composites were characterized. By utilizing the magnetic anisotropy properties of the printed composites, motions of the printed parts could be actuated at different positions in the applied magnetic field. Physical models were established to predict magnetic properties of the composite and trigger distance of fabricated parts. The predicted results agreed well with the experimental measurements, indicating the effectiveness of predicting macroscopic composite performance using microscale distribution data, and the feasibility of using the developed physical models to guide multimaterial and multifunctional composite design.

As an emerging and effective nanomanufacturing technology, the directional freezing-based three-dimensional (3D) printing can form 3D nanostructures with complex shapes and superior functionalities, and thus has received ever-increasing publicity in the past years. One of the key challenges in this process is the proper heat management, since the heat-induced melting and solidification process significantly affects the functional integrity and structural integrity of the printed structure. A novel approach for heat prediction out of modeling and optimization is introduced in this study. Based on the prediction, we propose a heuristic tool path planning method. The simulation results demonstrate that the tool path planning highly affects the spatial and temporal temperature distribution of the being printed part, and the optimized tool path planning can effectively improve the uniformity of the temperature distribution, which will consequently enhance the performance of the fabricated nanostructures.

Fresnel zone plates (FZPs) have been gaining a significant attention by industry due to their compact design and light weight. Different fabrication methods have been reported and used for their manufacture but they are relatively expensive. This research proposes a new low-cost one-step fabrication method that utilizes nanosecond laser selective oxidation of titanium coatings on glass substrates and thus to form titanium dioxide (TiO2) nanoscale films with different thicknesses by controlling the laser fluence and the scanning speed. In this way, phase-shifting FZPs were manufactured, where the TiO2 thin-films acted as a phase shifter for the reflected light, while the gain in phase depended on the film thickness. A model was created to analyze the performance of such FZPs based on the scalar theory. Finally, phase-shifting FZPs were fabricated for different operating wavelengths by varying the film thickness and a measurement setup was built to compare experimental and theoretical results. A good agreement between these results was achieved, and an FZP efficiency of 5.5% to 20.9% was obtained when varying the wavelength and the oxide thicknesses of the zones.

The method for fast fabrication of superhydrophobic surfaces was proposed to resist the formation of biofilm of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) for orthopedic and dental implants. Laser beam machining with nanosecond pulsed laser (Nd:YAG) was used to fabricate pit structure on Grade-5 Ti–6Al–4V alloy followed by annealing (at 300 °C with different time scales) in order to reduce the transition time from hydrophilic to superhydrophobic surface generation. Field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) techniques were used to characterize the textured samples. The surface wettability of plain and textured samples was measured by the sessile drop method using goniometer. The biofilm formation was qualitatively and quantitatively evaluated by FE-SEM and crystal violet binding assay, respectively. The biofilm formation was observed on plain (hydrophilic) surface for both the types of bacteria, whereas significantly less biofilm formation was observed on the laser textured (superhydrophobic) surfaces. The proposed method helps in reducing the risk of infection associated with implants without using cytotoxic bactericidal agents.

Thermal drawing from a preform recently emerges as a scalable manufacturing method for the high volume production of continuous metal microwires for numerous applications. However, no model can yet satisfactorily provide effective understanding of core diameter and continuity from process parameters and material properties during thermal drawing. In this paper, a long wavelength model is derived to describe the dynamics of a molten metal micro-jet entrained within an immiscible, viscous, nonlinear free surface extensional flow. The model requires numerical data (e.g., drawing force and cladding profile) be measured in real time. Examination of the boundary conditions reveals that the diameter control mechanism is essentially volume conservation. The flow rate of molten metal is controlled upstream while the flow velocity is controlled downstream realized by solidification of the molten metal. The dynamics of the molten metal jet are found to be dominated by interfacial tension, stress in the cladding, and pressure in the molten metal. Taylor's conical fluid interface solution (Taylor, 1966, “Conical Free Surfaces and Fluid Interfaces,” Applied Mechanics, Springer, Berlin, pp. 790–796.) is found to be a special case of this model. A dimensionless capillary number Ca=2Fa/γA(0) is suggested to be used as the indicator for the transition from continuous mode (i.e., viscous stress dominating) to dripping mode (i.e., interfacial tension dominating). Experimental results showed the existence of a critical capillary number Cacr, above which continuous metal microwires can be produced, providing the first ever quantitative predictor of the core continuity during preform drawing of metal microwires.

Reliability aspects are crucial for the success of every technology in industrial application. Regarding interconnect devices, several methods are applied to evaluate reliability of conductor paths like accelerated environmental tests. Especially, molded interconnect devices (MID), which enable numerous applications with three-dimensional (3D) circuitry on 3D shaped injection-molded thermoplastic parts are often under particular stress, e.g., as component of a housing. In this study, a new test method for evaluating the flexural fatigue strength of conductor paths produced by the laser-based LPKF-LDS® technology is presented. For characterization of test samples, a test bench for flexural fatigue test was built up. A result of the flexural fatigue test is a characteristic Woehler curve of the metal layer system. Applying this new test method, essential influencing parameters on the reliability of MID under mechanical load can be identified. So, the metal layer system as well as the geometric parameters of the metal layer is crucial for the performance. Furthermore, test specimens are tested under different types of mechanical load, i.e., tensile stress and compressive stress. For a holistic view on reliability of MID, experimental results are discussed and supported by simulations. An important finding of the study is the advantage of nickel-free layer systems in contrast to the Cu/Ni/Au layer system, which is often used in MID technology.

In this work, an electrohydrodynamic casting approach was used to manufacture a carbon nanofiber (CNF) composite material containing bismuth telluride (Bi2Te3) particles. A 10% polyacrylonitrile (PAN) polymer solution was taken as the precursor to generate nanofibers. Bismuth telluride microparticles were added into the polymer solution. The particle-containing solution was electrohydrodynamically cast onto a substrate to form a PAN-based nanofiber composite mat. High temperature heat treatment on the polymeric matrix composite mat in hydrogen atmosphere resulted in the formation of a microparticle-loaded CNF composite material. Scanning electron microscopic (SEM) analysis was conducted to observe the morphology and reveal the composition of the composite material. Energy conversion functions in view of converting heat into electricity, electromagnetic wave energy into heat, and photon energy into electricity were shown. Strong Seebeck effect, hyperthermia, and photovoltaics of the composite mat were found. In addition, the potential applications as sensors were discussed.

An output-only modal analysis (OMA) approach is presented to obtain the direct frequency response function (FRF) at the tip of the tool in micromilling setups. White noise input is provided using acoustic excitation and the resulting vibrations are measured using a laser Doppler vibrometer (LDV). Autoregressive (AR) identification is used to extract the natural frequencies and damping ratios of the structural modes of the milling setup, and mass-sensitivity analysis is used to obtain modal stiffness values. The accuracy of the tool tip FRFs that are constructed using OMA is verified by comparing them against the FRFs that are measured using impulse hammer tests. The direct FRF at the tool tip is an essential component in predicting and avoiding excessive and unstable vibrations in milling operations, and the presented approach provides a practical method for the direct measurement of the tool tip FRF in micromilling where the application of traditional hammer tests is not possible.

Bioceramics with porous microstructure has attracted intense attention in tissue engineering due to tissue growth facilitation in the human body. In the present work, a novel manufacturing process for producing hydroxyapatite (HA) aerogels with a high density shell inspired by human bone microstructure is proposed for bone tissue engineering applications. This method combines laser processing and traditional freeze casting, in which HA aerogel is prepared by freeze casting and aqueous suspension prior to laser processing of the aerogel surface with a focused CO2 laser beam that forms a dense layer on top of the porous microstructure. Using the proposed method, HA aerogel with dense shell was successfully prepared with a microstructure similar to human bone. The effect of laser process parameters on the surface, cross-sectional morphology and microstructure was investigated in order to obtain optimum parameters and has a better understanding of the process. Low laser energy resulted in a fragile thin surface with defects and cracks due to the thermal stress induced by the laser processing. However, increasing the laser power generated a thicker dense layer on the surface, free of defects. The range of 40–45 W laser power, 5 mm/s scanning speed, spot size of 1 mm, and 50% overlap in laser scanning the surface yielded the best surface morphology and microstructure in our experiments.

Technical Brief

Molecular dynamics (MD) simulations are used to gain insights into the process conditions that cause separation of graphene layers from a highly ordered pyrolytic graphite (HOPG) source in a polydimethylsiloxane (PDMS) stamp-assisted mechanical exfoliation process. Specifically, the effects of selected exfoliation process parameters and pre-existing defects, such as layer discontinuities in the graphite source, on the exfoliation process are investigated. The results show that exfoliation of individual and few layer graphene requires delicate control of the normal force applied to the HOPG by the PDMS stamp. The study also shows that defects (e.g., discontinuities) in the HOPG have a significant impact on the thickness of separated layers and the layer separation force. The insights derived from this study are expected to be very useful in the development of a low-cost, scalable, large area graphene production process.

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